Electrical power cords and wires have been identified by the U.S. Consumer Products Safety Commission as a leading cause of electrical fires. Problems in home wiring, like arcing and sparking are associated with more than 40,000 home fires each year. Electrical fires kill over 750 people, injure 1400 victims and cause over $1 billion in property damage annually.
Electrical power cords and permanent electrical wiring can fail in a variety of ways, resulting in localized heating, arcing, and combustion of materials surrounding the cords and wires. The two types of faults that are responsible for the majority of wiring-related electrical fires are: (1) series faults; and (2) parallel faults. In a series fault, a connection in series with the load is broke, such as breakage of conductor within its insulator occurs. Arcing may occur along the gap created by the breakage, resulting in localized heating.
In a parallel fault, a conduction path is created between the two conductors of the cord or wire, or between the phase or hot conductor and ground (ground fault), or both. A parallel fault develops in three distinct stages: (1) leakage; (2) tracking; and (3) arcing. Leakage currents occur normally and safely in any cord set and are related to the capacitance and the resistance of the insulation encasing the leads of the cord. As long as the insulating layer is in good condition, small leakage currents are considered harmless and safe. In the second stage, as the insulating layer degrades over time or becomes damaged, a conductive path may develop over the insulating surface. This is known as “tracking” and may actually accelerate insulation degradation. During the third stage (“arcing”) ionization of the air across the insulation gap occurs, providing a conduction path. The arc generates intense heat and can lead to combustion of surrounding materials. Degradation can also occur internally to the insulation medium due to excessive heat. Fire may start from either combustion of surrounding materials or the insulation itself, even in the absence of arcing.
A ground fault is type of parallel fault that is caused by leakage from a current-carrying conductor to ground, such as a short from the phase or hot conductor to the housing of an appliance. Even in the absence of arcing, if flammable material is present in or around the path to ground for leakage current, a fire hazard exists.
Leakage Current Protection Devices (LCPD) are a class of electrical or electromechanical devices for detecting leakage of current in an electrical circuit and include Ground Fault Interrupters (GFCI), Appliance Leakage Current Interrupters (ALCI), and Equipment Leakage Current Interrupters (ECLI). While GFCIs, APCIs, and ELCIs provide protection against shocks and fires by detecting arcing and faults to ground, these devices do not provide protection against across-the-line series and parallel arcing faults.
Arc fault and leakage current detection interrupters (AFCI/LCDI) protect against fires caused by arcing faults. In contrast to conventional circuit breakers that only respond to overloads and short circuits, AFCI/LCDIs are selective and protect against arcing conditions that produce erratic current flow. Normal arcing, i.e. from pulling a plug from a socket, power tool operation, or opening a switch, will not trigger an AFCI/LCDI. AFCI/LCDI circuitry continuously monitors flow through the circuit by utilizing current sensing circuitry to discriminate between normal and unwanted arcing conditions. The control circuitry trips internal contacts to de-energize the circuit when it detects an unwanted arcing condition.
What makes power supply cords used in connection with AFCI/LCDI circuits different from typical power supply cords is the following: (1) the hot and neutral conductors include a copper woven braid shield directly applied over the conductor insulation; and (2) the woven braid shield wire is coupled to an electrical circuit. The accepted standard for these types of power supply cords (UL Standard 758, Section 2) specifies a copper braid with an 85% minimum coverage applied directly over the conductor insulation. The fault detection device is coupled to the woven braid shield typically by solder termination. In this arrangement, the woven braid shield creates a circuit that can detect minute amounts of electrical current.
The typical function of the woven braid shield is to create the Faraday cage effect, which is useful in shielding against RFI and EMFI interference (radio frequency interference and electro magnetic frequency interference). The woven braid is used in conjunction with a foil tape for low and high frequency shielding effectiveness. Coaxial cable applications terminate the woven braid wire at the connector and control cables typically rely on a drain wire in conjunction with, and in contact with a foil shield, where the drain wire is used to “ground the circuit.” Both of these applications utilize a foil shield which provides 100% coverage and at the same time the foil helps to disperse the pressure applied by the braid wires not allowing them to impact the insulation thickness. The amount and volume of copper braid wire can be specified electrically by specifying a DCR (direct current resistance) value for the shield.
FIGS. 1 and 2 show an electrical cable 10 having conductors 12, 14 encased in woven-braid shields 16, 18. Each woven-braid shield 16, 18 is created through a typical braiding method where multiple bobbins (each with multiple ends) are placed on a braiding machine that passes one group of bobbin wires over the other in rapid succession—the top and bottom carriers of the braider move in opposite directions and deflector plates create the weave. The wires on each bobbin are under tension to maintain the weave, the tension is necessary to “close” the weave tight to the core. This tension is also necessary to allow the braid shield to maintain its position during subsequent processing. The combination of the bobbin wire tension and wire surface contact, braid wire “layer over layer”, allow the inside braid wires to penetrate or indent the conductor insulation surface. The “layer over layer” condition also creates shield redundancy, where strands cross each other or overlap that provides no additional shield detection or circuit capabilities.
In a power supply cord for appliances such as window air conditioners, use of relatively soft insulation material is desirable to improve product flexibility, i.e. the way the power supply cord “hangs” and handles after installation. The softness of the insulation creates an opportunity for the woven braid wire to significantly deform (in the range of 2-3%) the insulation material from wire tension and minimal point to point layer contact. This ultimately reduces the dielectric strength and electrical insulation properties of the insulation materials. The use of a woven braid wire exhibits a number of other drawbacks, including, but not limited to: (1) power supply cord weight and diameter, (2) difficulty of manufacture as a result of tensioning requirements during creation of the woven braid; and (3) complicated circuit termination as a result of the number and orientation of strands in the woven braid wire (as is seen in FIG. 2).
Accordingly, it is one object of the present invention to provide a power supply cord for use as part of an AFCI/LCDI circuit to detect conductor insulation breakdown, rupture or damage either between conductors or to an individual conductor, i.e. through the jacket and insulation, which addresses the drawbacks of currently available power supply cords.